Generally speaking, the term “SAW wave-guide” refers to bordered strips; and the term “wave-guide effect” relates to the effect which propagation along and within a SAW wave-guide has on a SAW beam as a result of the spreading SAW waves being reflected from the strip's borders.
For the purposes of the present application, the term “SAW wave-guide” should be understood as referring to a wide class of structural arrangements using electrode fingers that impart a SAW wave-guide effect to direct SAW beam propagated thereon, as well as to other kinds of transmitting SAW beams. For example, SAW wave-guides may also include electrode finger devices which transfer SAW beams from track-to-track, or to compress or expand SAW beams, or to reverse the direction of SAW beam propagation, etc. Hereinafter, the term “zebra-wave-guide” shall be understood as referring to a SAW wave-guide that has an appearance of zebra striping, i.e. it consists of electrode fingers laid perpendicular to the SAW wave-guide's longitudinal axis.
SAW devices are often employed as filters or resonators in high frequency applications. A SAW device contains a substrate of piezoelectric material upon which a propagating surface wave is accompanied by an electric field localized at the surface, thus enabling the wave to be controlled by configuring an array of metal electrodes on the surface. Input electrical signals applied to an input transducer are converted to surface acoustic waves propagating upon the surface of the substrate, and then reconverted from acoustic energy back to an electric output signal.
In certain circumstances, a SAW beam must travel a distance on the substrate that can be several hundred or even thousands of λ in length, where λ is the SAW wave-length at the interested frequency. Use of anisotropic piezo-substrate usually causes diffraction spreading and deflection of the SAW beams running through such a long propagation path and consequently, this, among other reasons, leads to insertion loss (i.e. diffraction loss) and distortion of the SAW device's response.
In order to either reduce or prevent diffraction spreading, SAW wave-guides are applied. The SAW wave-guide effect, i.e. ability of the wave-guide to preserve the energy of the SAW beam, is a result of the reflection of the SAW waves from the “borders” of the wave-guide. In one known design, the borders are constructed as wide bus bars and the wave-guide looks like a grating of equidistant, usually parallel electrically-shorted electrode fingers. Alternatively, the borders may appear like walls, giving the wave-guide the appearance of a groove in the surface of the substrate with no electrode fingers within. The inverse arrangement, where the area along both edges is depressed and separated by an elevated strip, is a variation of the walls structure. In another example, a wave-guide consists of an array of isolated electrode fingers, surrounded by conductor-free surface areas which serve as wave-guide borders.
U.S. Pat. No. 5,111,168 to Panasik, discloses the use of a zebra-type SAW wave-guide and the use of using the internal relief of the zebra-wave-guide to adjust the time delay of SAW beam propagation thereon. Mechanical and electrical boundary conditions are configured in alignment with the propagating piezo-acoustic surface wave's front. This design provides additional resistance against SAW beam spreading.
Certain kinds of multi-strip couplers (MSC), which are also forms of wave guides, for example, refracted 3 dB MSC or U-type 3 dB MSC, are only accurate at one frequency point, thereby limiting the possible application of such kinds of wave-guides.
It is also known to use SAW devices as coding devices, for example tags, see for example, “SAW tags. New ideas”—IEEE Ultrasonics Symposium—1995, pp. 117–120, by Plessky V. P., Kondratiev S. N., Stierlin R. and Nyffeler F. The following technique is known for controlling phase manipulation of such devices. Several parallel tracks are provided upon which SAW beams will be propagated. Each track has an initial part that is covered by a metal strip. Some of the strips are removed, for example, by photolithography etching process or by Laser Micro-Machining Systems, thereby providing a desired delay and phase shift of the associated SAW beam. It is thought that the delay and phase shift is due to the fact that the SAW velocity within the metal covered areas differs from the SAW velocity on the free surface space. The phase shift, achieved in this way, is further affected by the thickness of the metal layer.
SAW filters have inter-digital transducers (IDT). The inter-digital transducer (IDT) is an array of parallel fingers. The fingers have opposite polarities that are suitable for launching and/or detecting SAW waves. A simple periodical uniform IDT has a sin(x)/x passband shape which is not advantageous because its transition bandwidth is equal to the filter bandwidth, and more importantly, the first frequency sidelobe is typically only 13 dB below the main response. To synthesize arbitrarily-shaped passbands, IDT topology weighting is employed. Filtering is thus accomplished in the process of generating the surface acoustic wave by the input IDT, and in the inverse process of detecting the wave by the output IDT. The most effective filtering is preferably accomplished if both input and output IDTs are weighted, and thereby participate in the filtering process. Common IDT topology weighting techniques include apodization and withdrawal weighting. Apodization is typically used for wideband filters and either apodization or withdrawal weighting typically used for narrowband filters.
Apodization means varying the length of the electrodes to achieve electrode weighting. In order to achieve high precision band pass characteristics, an apodized IDT may have more than one thousand interdigitized fingers. However, further increasing the number of interdigitized fingers does not improve characteristics, because of numerous effects which are difficult to analyze, such as diffraction spreading and propagation loss that beginning to be understood as playing a substantial role.
It is well known that it is not practical to have an input apodized transducer launching a wave directly into an output apodized transducer, because an apodized transducer launches a wave, which has a non-uniform beam profile, and, as a receiving transducer, it expects to see a uniform beam profile. If a surface wave incident upon an apodized transducer is not uniform over the entire width of the beam, the frequency response changes dramatically. For this reason, one cannot use both input apodized and output apodized transducers to form a filter unless one adds a SAW wave-guide structure such as an MSC. The MSC, positioned between the apodized input and output transducers, transfers energy from a non-uniform beam into an adjacent track, in which a surface acoustic wave is launched as a uniform beam, and is thus compatible with an apodized transducer receiving the uniform beam. However, using an apodized input transducer for generating a surface acoustic wave, and transmitting the wave through an MSC to an apodized output transducer, widens the filter device, thus requiring increased space within electronic systems seeking to be ever more miniaturized.